Multiple Chromosome Fissions, Including That of the X Chromosome, in Aulacocyclus tricuspis Kaup (Coleoptera, Passalidae) from New Caledonia: Characterization of a Rare but Recurrent Pathway of Chromosome Evolution in Animals

The male karyotype of Aulacocyclus tricuspis Kaup 1868 (Coleoptera, Scarabaeoidea, Passalidae, Aulacocyclinae) from New Caledonia contains an exceptionally high number of chromosomes, almost all of which are acrocentric (53,X1X2Y). Unlike the karyotypes of other species of the pantropical family Passalidae, which are principally composed of metacentric chromosomes, this karyotype is derived by fissions involving almost all the autosomes after breakage in their centromere region. This presupposes the duplication of the centromeres. More surprising is the X chromosome fragmentation. The rarity of X chromosome fission during evolution may be explained by the deleterious effects of alterations to the mechanisms of gene dosage compensation (resulting from the over-expression of the unique X chromosome in male insects). Herein, we propose that its occurrence and persistence were facilitated by (1) the presence of amplified heterochromatin in the X chromosome of Passalidae ancestor, and (2) the capacity of heterochromatin to modulate the regulation of gene expression. In A. tricuspis, we suggest that the portion containing the X proper genes and either a gene-free heterochromatin fragment or a fragment containing a few genes insulated from the peculiar regulation of the X by surrounding heterochromatin were separated by fission. Finally, we show that similar karyotypes with multiple acrocentric autosomes and unusual sex chromosomes rarely occur in species of Coleoptera belonging to the families Vesperidae, Tenebrionidae, and Chrysomelidae. Unlike classical Robertsonian evolution by centric fusion, this pathway of chromosome evolution involving the centric fission of autosomes has rarely been documented in animals.

Wood fibers are a crucial microhabitat for cellulose- and xylan- degrading bacteria in the hindgut of the wood-feeding beetle Odontotaenius disjunctus

Introduction

Wood digestion in insects relies on the maintenance of a mosaic of numerous microhabitats, each colonized by distinct microbiomes. Understanding the division of digestive labor between these microhabitats- is central to understanding the physiology and evolution of symbiotic wood digestion. A microhabitat that has emerged to be of direct relevance to the process of lignocellulose digestion is the surface of ingested plant material. Wood particles in the guts of some termites are colonized by a specialized bacterial fiber-digesting microbiome, but whether this represents a widespread strategy among insect lineages that have independently evolved wood-feeding remains an open question.

Methods

In this study, we investigated the bacterial communities specifically associated with wood fibers in the gut of the passalid beetle Odontotaenius disjunctus. We developed a Percoll-based centrifugation method to isolate and enrich the wood particles from the anterior hindgut, allowing us to access the wood fibers and their associated microbiome. We then performed assays of enzyme activity and used short-read and long-read amplicon sequencing of the 16S rRNA gene to identify the composition of the fiber-associated microbiome.

Results

Our assays demonstrated that the anterior hindgut, which houses a majority of the bacterial load, is an important site for lignocellulose digestion. Wood particles enriched from the anterior hindgut contribute to a large proportion of the total enzyme activity. The sequencing revealed that O. disjunctus, like termites, harbors a distinct fiber-associated microbiome, but notably, its community is enriched in insect-specific groups of Lactococcus and Turicibacter.

Discussion

Our study underscores the importance of microhabitats in fostering the complex symbiotic relationships between wood-feeding insects and their microbiomes. The discovery of distinct fiber-digesting symbionts in O. disjunctus, compared to termites, highlights the diverse evolutionary paths insects have taken to adapt to a challenging diet.

Tonantzin, a New Genus of Bess Beetle (Coleoptera, Passalidae) from a Montane Subtropical Forest in Central Mexico, with a Review of the Taxonomic Significance of the Mesofrontal Structure in Proculini

Mexico has the third highest diversity of passalid beetles in the World. Here we describe Tonantzinnew genus, a new monotypic genus, potentially endemic to the mountains of central Mexico. The new genus is diagnosed by a new configuration of characters from the mesofrontal structure (MFS) in addition to other characters. The MFS in Passalidae has been treated either as a composite complex character or a combination of individual characters. Using a broad taxonomic sample within Proculini, we discuss the taxonomic and systematic implications of the MFS for the tribe. We define the MFS type tepetl. Given the importance of the MFS for passalid taxonomy we propose a new delimitation of the structure using boundaries based on internal and external head structures. We argue that the treatment of the MFS as a complex character better captures the nature of this structure but we ultimately find a need to standardize the way in which this structure is described in the taxonomic literature and used in phylogenetic analyses.

Phylogeny of the genus Yumtaax Boucher (Coleoptera, Passalidae, Proculini): Taxonomic and evolutionary implications with descriptions of three new species

Yumtaax Boucher (Coleoptera: Passalidae) is an endemic genus from the temperate sierras of Mexico and includes six narrowly distributed species. Yumtaax species have been assigned to several genera of Passalidae throughout history, and a phylogenetic approach is necessary to understand species delimitation and interspecific relationships. This study reconstructed the molecular phylogeny of six Yumtaax morphotypes using parsimony and Bayesian analysis of DNA sequence data from the ribosomal nuclear gene region 28S and the mitochondrial gene regions 12S and cytochrome oxidase I (COI) in addition to morphological characters. Analyses recovered two well-supported Yumtaax clades (the Yumtaaxlaticornis and Yumtaaximbellis clades) that are possible sister lineages. One synapomorphic morphological character state and the geographic isolation of the group provide corroborative evidence for monophyly. Molecular phylogenetic analyses and traditional morphological examinations also resulted in the discovery of two undescribed Yumtaax species and the discovery of two separate evolutionary lineages (cryptic species) within Yumtaaxrecticornis. As a result we describe three new species (Yumtaaxveracrucensis Beza-Beza, Reyes-Castillo & Jameson, sp. n., Yumtaaxcameliae Beza-Beza, Reyes-Castillo & Jameson, sp. n., and Yumtaaxjimenezi Beza-Beza, Reyes-Castillo & Jameson, sp. n.), redescribe two species (Yumtaaxrecticornis [Burmeister 1847] and Yumtaaxlaticornis [Truqui 1857]), and provide a key to all nine Yumtaax species. This study is one of two studies to use molecular data to evaluate the evolutionary relationships of a genus of Bess Beetles (Passalidae), an ecologically important insect group exhibiting low morphological variability and heretofore lacking molecular phylogenetic study.

Batwanema gen. n. and Chokwenema gen. n. (Oxyurida, Hystrignathidae), new nematode genera as parasites of Passalidae (Coleoptera) from the Democratic Republic of Congo

Two new genera and species parasitizing passalid beetles from the Democratic Republic of Congo are described. Batwanema congo gen. n. et sp. n. is characterized by having females with the cervical cuticle armed with scale-like projections, arranged initially in rows of eight elements that gradually divide and form pointed spines toward the end of the spiny region, two cephalic annuli, clavate procorpus and genital tract monodelphic-prodelphic. Two Malagasian species of Artigasia Christie, 1934 were placed in this genus as B. latum (Van Waerebeke, 1973) comb. n. and B. annulatum (Van Waerebeke, 1973) comb. n. Chokwenema lepidophorum gen. n. et sp. n. is characterized by having females with the cervical cuticle armed with scale-like projections, arranged initially in rows of eight elements (similar to Batwanema) that divide gradually, forming spines; a single cephalic annule cone-like, truncated, moderately inflated; procorpus sub-cylindrical and genital tract didelphic-amphidelphic.

Two new genera of nematode (Oxyurida, Hystrignathidae) parasites of Passalidae (Coleoptera) from the Democratic Republic of Congo

Two new genera and species parasitizing passalid beetles from the Democratic Republic of Congo are described. Kongonema meyerigen. n. sp. n. is characterized by having females with the cervical cuticle unarmed, first cephalic annule cone-like and truncate, sub-cylindrical procorpus and genital tract didelphic-amphidelphic. The males of Kongonema meyerigen. n. sp. n. have the procorpus sub-cylindrical, the dorsal cuticle of the tail end thickened, a single large, median mammiform pre-cloacal papilla and a pair of small, pre-cloacal, sub-lateral papillae at a short distance before the level of the cloaca. Lubanema decraemeraegen. n. sp. n. is characterized by the body markedly fusiform, cuticle unarmed and strongly annulated, procorpus sub-cylindrical, isthmus as a constriction between procorpus and basal bulb, genital tract monodelphic-prodelphic and the posterior end rounded with a very short tail appendage.

A new species of Oileus Kaup (Coleoptera, Passalidae) from Guatemala, with a key to the species of the genus

Oileus gasparilomisp. n. is described from nine specimens from the mountains of Quiché in Guatemala, from cloud forest at 1795 m elevation. This represents the second species of the genus for Guatemala, differing from the closely related Mexican species Oileus rimator (Truqui) in having a straight anterior clypeal border, the postocular sulcus divided by a keel and the pronotum almost straight (not clearly bilobed). A key to the species of Oileus is given.

On some Cuban species of the genus Longior Travassos & Kloss, 1958 (Oxyurida, Hystrignathidae), with description of a new species

Longior zayasi Coy, García & Alvarez, 1993 is established as incertae sedis because the males (declared as the holotype) are inconsistent with the generic diagnosis, particularly in relation to the morphology of the head and tail. Thus, the females of Longior zayasi species (which agree with the generic diagnosis) are renamed and re-described as Longior longior Morffe & García sp. n. We also described males found in the sample and considered as conspecific with the new species. A comparative table with the measurements of the most of the records of Longior longior is given. The male of Longior similis Morffe, García & Ventosa, 2009 is described from the type locality of the species and compared with the known males of the genus. A key to the females of the Cuban Longior is given.

Coynema gen. n., a new genus of nematode (Thelastomatoidea, Hystrignathidae) parasites of Passalidae (Coleoptera) from Cuba

The new genus Coynemagen. n. is described as parasite of the two passalid beetles from Cuba: Passalus interstitialis Escholtz, 1829 (type host) and Passalus pertyi Kaup, 1869. Females are characterized by the shape of their cephalic end, cervical cuticle unarmed, a sub-cylindrical procorpus with its base abruptly dilated, fore region of intestine dilated as a sac-like structure, genital system didelphic-amphidelphic and eggs markedly ovoid and smooth-shelled. Males have a digestive system similar to females, tail sharply pointed, bearing a Y-like thickening of the dorsal cuticle. They also present a big, median, mammiform pre-cloacal papillae and a pair of small, sub-dorsal pre-cloacal papillae anterior to the cuticular thickening of the tail.

Hystrignathus dearmasi sp. n. (Oxyurida, Hystrignathidae), first record of a nematode parasitizing a Panamanian Passalidae (Insecta, Coleoptera)

Hystrignathus dearmasi sp. n. (Oxyurida: Hystrignathidae) is described from an unidentified passalid beetle (Coleoptera: Passalidae) from Panama. It resembles Hystrignathus cobbi Travassos & Kloss, 1957 from Brazil, by having a similar form of the cephalic end, extension of cervical spines and absence of lateral alae. It differs from the latter species by having the body shorter, the oesophagus and tail comparatively larger, the vulva situated more posterior and the eggs ridged. This species constitutes the first record of a nematode parasitizing a Panamanian passalid.